The audio packs have two purposes. They add the ambient sounds for those that have the soldout version of TR (in which all the CD audio tracks are blank). Also they add extra little musical effects keying on events in the game (these effects were in the PS1 version of TR, but were never copied across to the PC version.
A problem with the audio packs is that when present they override all sounds that would come from the CD. At the moment this means you lose the sound for the cut scenes, because triggers for the cut scenes have yet to be created.
Having said that, it would be worth checking other threads here, because there has been some recent additional triggers created.